1. Field of Invention
The present invention concerns a method and its respective cooling system for a gas turbine; the invention also concerns a gas turbine comprising this cooling system.
2. Description of the Prior Art
A gas turbine comprises—in its main features—an axial compressor, several combustion chambers and an expansion turbine, enclosed within a pressurized box. Air from the atmosphere enters into the compressor and is compressed to be fed into the combustion chamber, where it reacts with a combustible to form a gas at high temperature (typically around 1000-1300° C.); the hot gas is then fed into the expansion turbine along an expansion course where it expands activating blades to transform its own thermal energy into mechanical energy.
In general, the materials with which the combustion chamber is built need a powerful cooler to avoid breakage. They are subjected to extremely high temperatures in this area of the machine. The cooling of the expansion turbine components directly subjected to the high temperature gas is also particularly critical for the efficiency and reliability of the machine.
In fact, the turbine's maximum distributable power basically depends on the maximum temperature attainable by the process gas which is substantially limited by the resistance of the materials of which the components in direct contact with the gas are made.
It is therefore extremely important to correctly cool these components to increase the maximum distributable power while decreasing the probability of breakage, which would be potentially catastrophic for the machine.
Note that the temperature of the process gas decreases along the expansion course, on which the components subjected to the highest temperatures are arranged upstream from the expansion course in proximity with the combustion chamber. In fact, if these components are not cooled, they deteriorate much faster than components not directly subjected to the hot gas, even when made with special materials with or without thermal barriers.
In particular, given the importance of these components arranged upstream from the expansion course, cooling techniques have been developed which are especially refined and at the same time costly to produce and maintain, such as film cooling, impingement, the use of forced turbulence or appropriate thermal barriers among others, as is well known by experts in the field.
Also note that the cooling system for components directly subjected to the gas at the highest temperature in general contributes to the regulation of the flow of cooling air; yet, to modify the cooling of one of these components, it is often necessary to make substantial modifications to the system or substitute the components; resulting therefore in increased maintenance and renovation costs.
On the other hand, there are components and areas of the expansion turbine that are not directly subjected to the process gas, and generally they do not require special materials for their construction and, if they are not cooled, they deteriorate less rapidly than the components directly subjected to the process gas. These components are therefore less critical and therefore the techniques used for their cooling are simpler and less expensive.
Currently then, despite technology developments, it is problematic and there is a need to make gas turbines that have ever-improving performance at relatively limited costs; these machines being projected in general to have better performance and resist the maximum temperatures attainable during full load operation conditions.